Various rubber compositions have been heretofore been prepared as masterbatches, or pre-formed rubber compositions, to prepare subsequent rubber compositions, and articles of manufacture, including tires, which have at least one component of a rubber composition comprised of such masterbatch-derived subsequent rubber composition.
For example, nanocomposites have been prepared which are composed of an elastomer which contains a dispersion of particles of intercalated clay which is at least partially exfoliated into individual platelets.
While relatively bulky, substantially hydrophilic, clay itself is normally viewed as a rubber diluent rather than reinforcement, clay particles in a form of more hydrophobic, very small, clay platelets may be considerably more efficient for the reinforcement of rubber compositions.
Indeed, it is to be appreciated that good dispersions of clay particles are somewhat difficult to obtain by simply mixing the clay with diene-based rubber compositions because the clay particles, in general, are primarily hydrophilic in nature and therefore are less compatible with, and tend to repelled by, diene-based elastomers. Should very small exfoliated clay platelets derived from water-swellable clay particles be desired for use as reinforcement in an elastomer composition, then it is considered herein that the better the dispersion of the particles within the elastomer the better the reinforcing effect. Accordingly, it is considered herein that a better dispersion is accomplished by blending exfoliated clay particles with one or more elastomers in a form of a nanocomposite thereof in which the very small exfoliated clay platelets have been rendered more hydrophobic in nature and therefore more compatible with diene-based elastomers by the intercalation and exfoliation thereof in situ within an anionic emulsion of elastomer particles by use of an amine having at least two amine centers as a cationic polymeric quaternary amine or ethylene polyamine via an ion exchange with cation exchangeable ion(s) contained within the galleries of the layers of the water-swellable clay within the anionic emulsion of elastomer particles instead of simply dry-mixing the clay with an elastomer composition.
In one aspect of this invention, a masterbatch-derived rubber composition is provided as such a nanocomposite, where the nanocomposite may be referred to herein as a masterbatch, and where the masterbatch nanocomposite, with its dispersed partially exfoliated, intercalated clay particle content, is blended with at least one additional elastomer to form the resultant rubber composition.
In another aspect of this invention, an article of manufacture is provided, such as for example tires and industrial product such as, for example, conveyor belts, power transmission belts and hoses, which has at least one component of a rubber composition comprised of the nanocomposite or comprised of said rubber composition which contains such nanocomposite (or masterbatch if the nanocomposite is referred to as a masterbatch).
It is considered herein that such rubber composition containing such nanocomposite, when used to replace a portion of the normally used carbon black reinforcement, may be particularly adaptable for use as a component for a tire, particularly a tire tread such as an aircraft tire tread. Reduced hysteresis (e.g. an increase in 100° C. rebound value or a reduction in tangent delta value) is desired to promote a reduction in heat buildup of the tire, and therefore an increase in tire durability. This compositional characteristic is especially important due to relatively high speeds and loads aircraft tires experience during take-offs and landings. The weight reductions possible by replacing a portion of the carbon black reinforcement of the rubber composition can promote an increase in the aircraft vehicular fuel economy both on the ground and in the air. The reduction in component weight combined with the aforesaid promoted tire durability also allows the tire manufacturer greater flexibility in tire design to make the overall tire more durable, especially in applications where tire weight is severely limited by the airframe manufacturer's design of the aircraft itself.
Nanocomposites composed of elastomers and a dispersion of particles of intercalated, and possibly partially exfoliated, water-swellable clay have heretofore been prepared by various methods.
For example, such nanocomposites have been prepared by first pre-intercalating a multi-layered, hydrophilic water-swellable clay in water which contains an intercalating compound (e.g. a quaternary ammonium salt) to intercalate the clay by causing an ion exchange to occur in which the quaternary ammonium salt displaces one or more cations contained within the galleries of the multi-layered clay. The resultant intercalated clay particles are dried and then mixed with an elastomer to form a dispersion thereof within the elastomer. To a small extent, the layers of the intercalated clay may become delaminated, or exfoliated, into individual platelets, which may include delaminated, or exfoliated, stacks of platelets, either during the intercalation process or upon subsequent high shear mixing with the elastomer.
Such a method is considered herein to be excessively dependent upon high shear mixing of the intercalated clay into the elastomer composition and relatively inefficient insofar as obtaining a good overall dispersion of substantially exfoliated platelets of an intercalated water-swellable clay within an elastomer matrix and therefore not likely to be relatively cost efficient method of nanocomposite preparation.
Other suggested methods of nanocomposite preparation include, for example, utilizing an ion exchange phenomenon between cation exchangeable ions contained within the galleries of stacked platelets of a water-swellable clay composed of multiple layers of negatively charged stacked platelets and cationically (negatively) charged elastomer particles contained in an aqueous latex thereof. By such method the exfoliated platelets are thereby contemplated as being created in situ within the latex.
In practice, a maximized state of exfoliation of the clay into individual platelets is considered herein to be desirable in order to enhance reinforcement of elastomer-based components of articles of manufacture, particularly tires and more particularly tire treads.
It is therefore desired herein to provide a significantly exfoliated, intercalated, water-swellable clay in or from a relatively low shear medium, for example a latex, prior to dry blending under high shear conditions with an elastomer composition.
Accordingly, for this invention, a process of creating a dispersion of exfoliated clay platelets in an elastomer is provided which is considered herein to be a significant departure from past practice.
In practice, for this invention, a dispersion of at least partially exfoliated, intercalated, water swellable clay particles in an elastomer is provided by blending a water slurry of water-swellable multilayered clay (e.g. a smectite clay) with an emulsion (latex) latex of anionic (negatively charged) elastomer particles having a pH in a range of about 6 to about 10 and blending therewith an amine having at least two amine centers as a cationic (positively charged) polymeric quaternary amine or ethylene polyamine to effect an ion exchange with cation exchangeable ion(s) contained within the galleries of the stacked platelets of the clay and thereby intercalate the clay and cause at least a partial exfoliation of the clay into individual clay platelets, all in situ. In practice, a small amount of acid, or salt/acid combination, is added to reduce the pH of the emulsion to a value, for example, in a range of from about 3 to about 4, to aid in coagulating (precipitating) the elastomer particles and partially exfoliated, intercalated, clay as a nanocomposite.
The nanocomposite may then simply be recovered by drying the coagulant, or precipitate.
In practice, the anionic (negatively charged) elastomer particles of synthetic elastomers may be formed, for example, by use of anionic surfactant(s) to stabilize the emulsion. Such use of anionic surfactants for such purpose is well known to those having skill in such art.
In practice, an acid, or salt/acid combination, often is used to reduce the pH of an anionic latex from a pH, for example, in a range of about 6 to about 10 to a more acidic value in a range of, for example, of about 3 to about 4 to therefore promote a destabilization of the emulsion and promoting a coagulation, or precipitation, of the elastomer particles from the emulsion. A representative example of an acid, or salt/acid combination, for such purpose is, for example, sulfuric acid or a combination of sodium chloride and sulfuric acid. Such use of an acid, or salt/acid combination is well known to those having skill in such art.
In practice, a coagulation promoting agent for the elastomeric particles in an anionic emulsion may be, for example, a cationic liquid polymeric quaternary amine. A representative example of material containing a cationic polymeric quaternary amine sometimes used for such purpose is, for example, Perchem 503™ from the Petrolite Company. A representative example of an ethylene polyamine as a mixture of ethylene polyamines for such purpose is, for example, PM-1969™ from the Union Carbide Company.
However, the presence of such cationic polymeric quaternary amine in the residual, recovered elastomer may also serve as a sulfur vulcanization accelerator in a sulfur vulcanizable rubber composition which contains the recovered elastomer.
Accordingly, excessive use of such cationic polymeric quaternary amine for coagulation of the anionic elastomer particles from the latex is considered herein to be undesirable because its content within a sulfur curable diene-based elastomer (e.g. styrene/butadiene copolymer elastomer) is considered herein to unsatisfactorily accelerate the sulfur curing, or vulcanization, of diene-based elastomers.
For this invention, however, it is considered herein that a resultant synergistic combination of the water-swelled clay and cationic (positively charged) polymeric quaternary amine inclusion in an aqueous emulsion of anionic (negatively charged) elastomer particles results.
While one aspect of the mechanism might not be fully understood, it appears that a more acceptable content, or effective content, of the resultant polymeric quatemary amine is contained in the recovered nanocomposite in that it apparently has less sulfur vulcanization acceleration effect in a sulfur-containing rubber composition which contains the nanocomposite perhaps because of a reduction in the amount of the polymeric quaternary amine needed to effect the aforesaid elastomer particle coagulation or perhaps the polymeric quatemary amine is combined with the aforesaid intercalated clay in a manner that its sulfur vulcanization acceleration is attenuated.
In this manner, then, it is considered herein that both a more efficient use of a cationic polymeric quatemary amine coagulant for the elastomer from the emulsion is enabled and, also a more efficient in situ formation of a reinforcing material in the resultant elastomer is enabled in a form of an at least partially exfoliated, intercalated clay.
Further, the aforesaid use of an acid, or salt/acid combination, can be beneficially used to aid in the coagulation process by reduction of the pH of the emulsion/clay mixture in combination with, or together with, the addition of amine having at least two amine centers as the cationic polymeric quaternary amine or ethylene polyamine for coagulation, or precipitation, of the elastomer and clay particles from the anionic emulsion mixture. Thus, in one aspect, the addition of the acid, or salt/acid combination, may, in one respect, be considered a part of the synergistic procedure.
Accordingly, this invention is considered herein to be a significant departure from past practice by a synergistic blending of a minor amount of an amine having at least two amine centers as a cationic polymeric quatemary amine or ethylene polyamine with an aqueous mixture of anionic elastomer particles and water swelled, water-swellable clay, together with an acid, or salt acid combination, to effect both a coagulation/precipitation of the elastomer/clay particle composite and an in situ formation of reinforcement for the elastomer of partially exfoliated, intercalated clay particles.
Therefore, a significant aspect of this invention is the intercalation of the water-swelled clay contained in an anionic emulsion of elastomer particles, wherein the water-swelled clay contains cation exchangeable ions (e.g. sodium ion) within the galleries between its platelets and wherein the intercalation is accomplished by addition of an amine having at least two amine centers as a cationic polymeric quatemary amine or ethylene polyamine to effect an ion transfer between the ions within the clay galleries and the cationic quatemary amine or ethylene polyamine.
A further significant aspect of the invention is the substantially simultaneous precipitation (coagulation) of the elastomer with contained dispersion of the intercalated (and partially exfoliated) clay particles as a nanocomposite which is aided by the addition of the acidic water to destabilize the emulsion.
Thus, the practice of this invention excludes a sole use of an acid, or salt/acid combination, to destabilize the anionic emulsion and coagulate/precipitate the elastomer/clay composite from the emulsion mixture.
In an additional departure from past practice, the water-swellable clay is introduced into the emulsion of anionic elastomer particles in a pre-water swelled from but without being first intercalated with an intercalant (e.g. a non-pre-intercalated clay as being a water-swelled clay which is not first intercalated with a quaternary ammonium salt to effect an ion exchange prior to its addition to the emulsion) so that the addition of the cationic polymeric quaternary amine or ethylene polyamine to the emulsion/clay mixture is relied upon to intercalate the water-swelled clay by the aforesaid ion exchange in situ within the emulsion of anionic elastomer particles.
For the practice of this invention, it is intended that the clay intercalation and exfoliation process for this invention is conducted in the presence of the anionic (negatively charged) elastomer particles to an exclusion of a thermoplastic polymer latex and to the exclusion of cationic (positively charged) elastomer particles, particularly cationic elastomer articles contained in a cationic surfactant.
In a summary, then, the process of this invention differs significantly from past practice, at least in part because the water-swellable clay (e.g. smectite clay) is
(A) not intercalated during the polymerization of the monomers,
(B) not intercalated by physically blending the smectite clay with the elastomer after it has been coagulated and recovered as a dry elastomer and
(C) not intercalated by blending a smectite clay which has been pre-intercalated by treatment with a quaternary ammonium salt prior to blending the pre-intercalated clay with the elastomer.
Thus, it is readily seen, and it is considered herein, that the process of this invention differs significantly from a relatively simple past practice of coagulating a latex emulsion with a polymeric quaternary amine.
Indeed, while some elements of the process of this invention might appear to be somewhat simplistic in operational nature, it is considered herein that the overall technical procedural application is a significant departure from past practice.
Water-swellable clays considered for use in this invention which are clays composed of a plurality of stacked platelets (e.g. very thin silicate based platelets) which contain cationically exchangeable ions in the galleries between such platelets. Representative of such clays are water swellable smectite clays, vermiculite based clays and mica based clays. Preferably such water-swellable clays are smectite clays. Representative of smectite clays are, for example, montmorillonite, hectorite, nontrite, beidellite, volkonskoite, saponite, sauconite, sobockite, sterensite, and sinfordite clays of which montmorillonite and hectorite clays are preferred. For various exemplary smectite clays, see for example U.S. Pat. No. 5,552,469. Such cationically exchangeable ions contained in such galleries are typically comprised of at least one of sodium ions and potassium ions, which may also include calcium ions and/or magnesium ions, although it is understood that additional cationically exchangeable ions may be present. Typically, montmorillonite clay is preferred which contains sodium ions in such galleries, although it is understood that a minor amount of additional cationically exchangeable ions may be contained in such galleries such as for example, calcium ions.
In practice, the degree of exfoliation of the intercalated clay platelets can be qualitatively evaluated, for example, by wide angle X-ray diffraction (WAXD) as evidenced by a substantial absence of an X-ray peak which is a well known method of such evaluation. Such evaluation relies upon observing WAXD peak intensities and changes (increase) in the basal plane spacing between platelets.
It is to be appreciated that, in practice, a synthetic emulsion of anionic elastomer particles may be prepared, for example, by emulsion polymerization of monomers selected from, for example, styrene and 1,3-butadiene or 1,3-butadiene, or 1,3-butadiene and acrylonitrile, or styrene and isoprene, or isoprene, and particularly the styrene and 1,3-butadiene monomers, in a water emulsion medium via a free radical polymerization initiators in the presence of an anionic surfactant. Preferably the monomers are a combination of styrene and 1,3-butadiene to form anionic styrene/butadiene copolymer elastomer particles in the emulsion.
It is also to be appreciated that the emulsion, or latex, of anionic elastomer particles may be natural cis 1,4-polyisoprene contained in a natural rubber latex.
Representative examples of anionic surfactants for the preparation of the synthetic emulsion of anionic elastomer particles may be found, for example, in McCutcheon's, Volume 1, “Emulsifiers & Detergents”, North American Edition, 2001, Pages 291 and 292, with representative examples of non-ionic surfactants shown on Pages 294 through 300 and examples of cationic surfactants shown on Pages 300 and 301.
For the practice of this invention, cationic surfactants for the preparation of the synthetic elastomer particles are to be excluded.
In one aspect, a water swellable clay, such as for example a smectite clay such as, for example, a montmorillonite clay, for use in this invention, might be described, for example, as a naturally occurring clay of a structure which is composed of a plurality of stacked, thin and relatively flat, layers, where such individual layers may be of a structure viewed as being composed of very thin octahedral shaped alumina layer sandwiched between two very thin tetrahedrally shaped silica layers to form an aluminosilicate structure. Generally, for such aluminosilicate structure in the naturally occurring montmorillonite clay, some of the aluminum cations (Al+3) are viewed as having been replaced by magnesium cations (Mg+2) which results in a net negative charge to the platelet layers of the clay structure. Such negative charge is viewed as being balanced in the naturally occurring clay with hydrated sodium, lithium, magnesium, calcium and/or potassium cations, usually primarily sodium ions, within the spacing (sometimes referred to as “galleries”) between the aforesaid aluminosilicate layers, or platelets.
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” or “uncured”, unless otherwise indicated.